Method and a device for the refining of glass

ABSTRACT

A device for the refining of a glass melt at high temperatures according to the skull pot principle is provided. The device includes a skull crucible having walls that are constructed from a plurality of pipes, a high-frequency coil for coupling electrical energy into the contents of the skull crucible, and an inlet and an outlet of the skull crucible being arranged in a melt surface region of the glass melt, wherein the inlet and the outlet are essentially arranged lying opposite one another.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 10/362,396 filed Jul. 16, 2003, now pending, which is a nationalstage entry of International Application No. PCT/EP01/08148 filed onJul. 14, 2001, which claims the benefit of German Application No. DE 10041 757.4-45 filed on Aug. 25, 2000, the entire contents of all of whichare incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns the production of glass in general from wasteglass or glass batches. The three essential stations of the productionprocess comprise melting, then refining and finally homogenizing.

2. Description of Related Art

The production of high-value special glasses requires the process stepof refining after melting, in order to remove the residual bubbles fromthe melt. The prior art comprises the refining of glasses by addition ofrefining agents such as redox refining agents or evaporating refiningagents. One speaks here of chemical refining, since the release of gasesform the melt is utilized in order to inflate small bubbles that arepresent and thus to facilitate the rise of these bubbles.

Along with the methods of chemical refining, alternatively oradditionally, physical effects are utilized, as described in theliterature, for expelling bubbles and thus for refining, such as, forexample, centrifugal force (U.S. Pat. No. 3,893,836) or the reduction ofthe bath depth and thus the rise of bubbles to the surface of the meltis facilitated (DE 197 10 351 C1).

It is known that refining is promoted by increasing the temperature ofthe melt. However, when refractory material is used for the refiningtank, limits are imposed. If ceramics with high a zirconium content areused, then temperatures of a maximum 16,500° C. can be produced.

It is known also to conduct refining in an apparatus that operatesaccording to the so-called skull pot principle. See EP 0 528,025 B1.Such a device comprises a crucible, the walls of which are formed from aring or collar of metal pipes, which can be connected to a coolingmedium, with slots between the metal pipes adjacent to one another. Thedevice also contains an induction coil, which surrounds the walls of thecrucible and by means of which high-frequency energy can be coupled intothe contents of the crucible. This direct heating of the glass melt bymeans of irradiation of high-frequency energy is conducted at a power of10 kHz to 5 MHz.

Such a crucible permits essentially higher temperatures than a vesselmade of refractory material. The advantage of high-temperature refiningin comparison to all other physical refining processes is that it isvery effective and rapid due to the high temperatures. The processestake place clearly more rapidly at high temperatures, so that verysmall, rapid aggregate modules can be prepared for the process ofrefining.

DE 2,033,074A describes an arrangement for the continuous melting andrefining of glass. A refining device is provided therein, which operatesaccording to the skull pot principle. The melt from the bottom region ofthe melting vessel reaches the refining vessel via a connection channel.It enters in the bottom region of the latter. The glass flow in therefining vessel thus rises upward from the bottom. This has theadvantage that the flow has the same direction as the lifting force ofthe bubbles. The bubbles to be removed reach the hot surface of the meltand are discharged from the latter.

A disadvantage of this embodiment consists of the fact that theconnection channel between the melting-down basin and the high-frequencyrefining device is subject to intense wear and tear due to the high flowvelocities.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to develop a system in which the goodrefining results remain, based on an upward flow of the glass melt, butin which also the melt remains hot at the surface in the region wherethe bubbles are discharged, so that all bubbles can burst at thesurface, and in which the problematic connection channel between themelting vat and the refining device can be omitted.

The inventor has recognized the following: If the inlet as well as theoutlet of the high-frequency crucible is arranged in the upper regionand in fact in such a way that the two of these lie opposite oneanother, then a very good and effective refining results. One would haveexpected that with such a structure, an essential part of the melt wouldbe unheated and unrefined and led along directly to the outlet in theshort circuit from the inlet at the surface. However, this is not thecase. Rather, a defined flow is set up based on the differences indensity in different melt regions. If the expansion coefficient of themelt is sufficiently high and the heating of the melt in the crucible isassured appropriately, the laterally introduced cold glass does notdirectly reach the crucible outlet via short-circuit currents, but isfirst pulled to the bottom of the crucible and from here is led to thesurface and to the outlet via convection rollers according to circularmovements of variable length.

The inlet and outlet should essentially lie diametrically opposite eachother. This is not absolutely necessary, however; certain deviations areadmissible. Also, the crucible should be dimensioned correctly, but thisis an optimizing problem, which can be solved by the person of averageskill in the art.

The connection channel between the melting vat and the refiningcrucible, which is known from the prior art, will be avoided. Instead ofthis, the melt can overflow from the melting vat into an open channel tothe refining crucible.

It may be appropriate to configure the refining crucible according to DE2,033,074 A. The crucible comprises a lower part of relatively smalldiameter, and an upper part of relatively large diameter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention is explained in more detail on the basis of the drawing.The following are shown individually therein:

FIG. 1 shows a set-up for the production of glass.

FIG. 2 shows a refining crucible according to the invention in avertical section.

FIG. 3 shows another embodiment of a set-up for the production of glass.

FIG. 4 shows a cooled bridge barrier in the skull crucible, in schematicrepresentation.

FIG. 5 illustrates the integration of the bridge barrier into a skullcrucible.

FIG. 6 shows a set-up for the melting of glass with two refiningstations.

DETAILED DESCRIPTION OF THE INVENTION

The set-up shown in FIG. 1 comprises a melting-down basin 1 with anintroduction device 1.1. The glass batch 1.2 which has been introducedis retained by a bridge barrier 1.3 to keep it from flowing further tothe stations connected downstream.

An overflow channel 2 is connected to the melting-down basin 1. This isopen at the top. The crude melt reaches a refining device 3 via theoverflow channel 2.

This refining device comprises a skull crucible and also ahigh-frequency coil, which is not shown here. The actual refining isconducted here at temperatures of 1750 to 3,000° C., depending on theglass synthesized and the requirements for glass quality.

After the refining, the melt is free of bubbles. It reaches ahomogenizing device 5, which in turn comprises a stirring crucible and astirrer, via a conventionally heated channel system 4.

The structure of the skull crucible can be recognized in detail in FIG.2. This involves a so-called mushroom skull crucible according to DE2,033,074 A. The skull crucible has a lower crucible part 3.1 of arelatively small diameter, and in addition an upper crucible part of arelatively large diameter. The upper crucible part also contains theinlet 3.2 and the outlet 3.3 for the melt. The arrows indicate the flowof the melt. As is seen, the cold glass introduced laterally through theinlet 3.2 first falls downward to the bottom of the crucible 3.4, thenrises again upward in order to once more flow downward and then upwardagain. As is seen, the lower part 3.1 of the skull crucible issurrounded by a high-frequency coil 3.5.

The set-up shown in FIG. 3 is the refining device 3 equipped with anadditional, cooled bridge barrier 3.6. This has the following task: Ifthe glass arriving in the skull refining aggregate is very foamy or theexpansion coefficient of the melt as a function of temperature is verysmall, then the danger exists that a small portion of the melt is drawnover the surface. This can be prevented either by a clear increase inthe temperature difference between the melt flowing in and the melt inthe core of the crucible in the skull crucible module or byincorporating the bridge barrier 3.6.

The bridge barrier 3.6 may be comprised of either a gas-cooled orliquid-cooled ceramic material or of a water-cooled metal material.Modifications of cooled metal components lined with ceramics are alsoconceivable. If the bridge barrier has metal components, which lie abovethe surface of the melt and come into contact with the burneratmosphere, then it may be helpful to coat the bridge barrier with athin layer of Teflon (<150μ) in order to prevent a corrosion of themetal surface due to the aggressive burner atmosphere. The bridgebarrier 3.6 can either be positioned centrally in the refining module orcan be laterally displaced to inlet 3.2. The latter modification has theadvantage that the hot zone where the bubbles rise can be made as largeas possible. If the bridge barrier is constructed of metal material,then it should be electrically connected to the metal skull crucible, sothat no induced voltages build up between the metal corset and thebarrier, since these can lead to arcing and thus to the disruption ofthe metal wall. If an electrical connection cannot be produced, then allcomponents must be operated in an electrically free-floatingmanner—i.e., not grounded. This is particularly possible if the melttends toward intense crystallization, since In this case a stablepuncture-proof intermediate layer is formed, which reliably stops thearcing.

An example of embodiment of such a bridge barrier 3.6 is shown in FIG.4. The incorporation of such a barrier 3.6 can be seen in FIG. 5. Here,the barrier 3.6 is positioned below the surface of the melt. This hasthe advantage that there are no cold metal components in the upperfurnace space. The condensation of burner off-gases is particularlyproblematical on cold components. It is a disadvantage in this type ofassembly that large fluctuations in the glass level cannot be allowed,since in order to assure that no liquid melt flows over the barrier, theimmersion depth should be a maximum of 1 cm below the surface of themelt.

A barrier assembly can be made possible with the edge of the barrierabove the upper edge of the glass bath by lining the metal barriereither with Teflon or ceramic materials or by raising the glass levelfirst higher at the beginning of the process—and in fact raising it overthe upper edge of the barrier—and then again lowering the glass level tothe normal level in operation. In this case, a glazing of the barrier isachieved, which protects the barrier from attacks due to burneroff-gases. In addition to the embodiment of the barrier that is shownhere, simpler embodiments, for example, a simple ceramic stone barrieror even a cooled metal rod which runs crosswise over the crucible isconceivable.

An electrical connection 3.7 of the crucible 3 with the barrier 3.6 aswell as a crucible short-circuit ring 3.8 can be seen in detail in FIG.5.

A cascade refining is provided In the set-up shown in FIG. 6. Theintroduced glass batch 6 as well as a bridge barrier 7 can also berecognized again here.

Several refining modules are connected one after the other and theyconnect with one another simply in the upper region. The connectionsites can be heated conventionally, for example, with burners. In thiscase, complicated connection channels that are sensitive to disruptionand consume a great deal of energy can be omitted. An example with tworefining modules connected one after the other is shown in FIG. 6. Ofcourse, any number of refining modules connected one after the other isconceivable.

With respect to geometry—particularly diameter—, HF-frequency and HFvoltage are adapted to the conductivity of the glass to be melted ineach case. If different types of glass with clearly different electricalconductivities are to be melted in the same vat and are to be refined bymeans of HF heating, then this is not possible without retrofittingmeasures (connection of another generator with adapted frequency region,connection of an adapted coil, possible change of the melting diameter,adaptation of the capacities in the HF generator). Of course, as in FIG.6, two or more aggregates can be connected one after the other, and thuseach individual module can be adapted to different electrical meltingproperties. The HF energy is only turned on in the HF refining moduleadapted to the respective melt, whereas the other modules are not heatedwith HF energy, but only with conventional energy—such as, for example,burners in the upper furnace space. The melt flows over the modules thatare not turned on and is drawn into and heated only in the HF-heatedmodule. In order to configure the exchange of glass in such an aggregatein a simpler and quicker manner, it is helpful if each module has anadditional bottom outlet 9, which is opened for a short time in theglass exchange phase. Such a bottom outlet can also be of use in thecase of the simple structure with only one HF-module—particularly ifexchanges of glass in the vat are considered—but also if bottom residuesshould deposit thereon.

Another advantage of the invention is the very good “emergency runningproperties” of the set-up if there are disruptions in the HF range. Ifthe high-frequency heating apparatus fails for any reason whatever, thenthere exists the danger of a freezing up of the continuous flow in thecase of the continuous-flow crucible with introduction from below,whereby the glass flow is interrupted. The danger does not exist inprinciple in the present invention, since the glass flow can be assuredin each case by utilizing the upper heat of the burner.

1. A device for the refining of a glass melt at high temperaturesaccording to the skull pot principle, comprising: a skull cruciblehaving walls that are constructed from a plurality of pipes; ahigh-frequency coil for coupling electrical energy into the contents ofthe skull crucible; and an inlet and an outlet of the skull cruciblebeing arranged in a melt surface region of the glass melt, wherein theinlet and the outlet are essentially arranged lying opposite oneanother.
 2. The device according to claim 1, further comprising two ormore skull crucibles connected in series.
 3. The device according toclaim 1, further comprising a bridge barrier provided in the meltsurface region.
 4. The device according to claim 3, wherein the skullcrucible is mushroom-shaped and has a lower part of relatively smalldiameter as well as an upper part of relatively large diameter, andwherein the inlet and the outlet are connected in the upper part of theskull crucible.
 5. The device according to claim 3, further comprisingtwo or more skull crucibles connected in series.
 6. The device accordingto claim 1, wherein the skull crucible is mushroom-shaped and has alower part of relatively small diameter as well as an upper part ofrelatively large diameter, and wherein the inlet and the outlet areconnected in the upper part of the skull crucible.
 7. The deviceaccording to claim 6, further comprising two or more skull cruciblesconnected in series.